590 FERTILIZATION 



it occurs during the division of the zygote. Pascher (1916), in his work 

 on Mendehan inheritance in Chlamydomonas, presented genetic evidence 

 that reduction is zygotic — that is, reduction in chromosome number 

 occurs in the first two divisions of the zygote, which produce four 

 swarmers. This would mean that only the zygote is diploid and that all 

 other stages in the life cycle are haploid (see p. 611 below) . 



Here, indeed, is a peculiarly promising opportunity for the correla- 

 tion of cytological and genetic evidence of chromosome behavior, if 

 only the cytological data were available. Meiosis, not complicated by 

 subsequent fusion of gametes, and the attendant bringing together of 

 homologous chromosomes would offer some interesting possibilities. 



Another excellent example of a series of organisms exhibiting pro- 

 gressively advancing stages in the evolution of sex is found in the colonial 

 phytomonad flagellates. This series is so well known that it is usually 

 discussed even in textbooks on general biology. 



At one end of the series is Gon'ium pectorale. At certain times the 

 sixteen cells making up this flat colony function as gametocytes by pro- 

 ducing isogametes, which copulate in pairs to form zygotes. The gametes 

 may vary somewhat in size, but the manner in which they copulate — 

 apparently at random with any of the others — indicates that the slight 

 variation in size of the gametes is without significance. In Stephano- 

 sphaera pluvialis, a colony of eight cells, the gametes are all identical. 

 The chief advance which these isogamous Volvocidae exhibit over the 

 Copromonas type of fertilization is that the gametes are different from 

 the vegetative form. In other words, vegetative or asexual forms may 

 be distinguished from sex cells. 



In PandorJna morum (Pringsheim, 1869), a subspherical colony of 

 sixteen cells, two distinct sizes of gametes are produced, and two combi- 

 nations are possible. Small gametes may fuse with other small gametes, 

 and small gametes may fuse with large ones. Large gametes, however, 

 never fuse with other large ones. Here, then, in a single species is ex- 

 hibited both isogamy and heterogamy, for the failure of the large gam- 

 etes to fuse with each other indicates that the size difference is significant. 

 The critical factor may be the size itself, or it may be some less obvious 

 factor associated with size. 



It might be argued from this that primitive heterogamy is associated 

 with hermaphroditism. The same colony produces both large and small 



